RF circuits are well known in the art to produce, amplify and modulate radio frequency energy. RF energy radiating from the RF circuits is also known to increase the noise level that, in turn, affects the operation of adjacent electronic circuits. For example, the operation of adjacent electronic circuits becomes erratic and unpredictable as the RF energy, and the resultant noise level, ebbs and flows from the RF circuits. This RF interference, also known as electromagnetic interference (EMI), is an extremely important factor in determining the proper performance and functionality of electronic circuits adjacent to RF circuits. Accordingly, it is necessary to confine radiating RF energy to protect sensitive electronic components and prevent significant RF energy from radiating from a product.
One method to protect sensitive components from RF interference is to place RF shields around critical electronic components. An RF shield is a continuous conductive structure that surrounds and encloses components to prevent RF radiation from entering, leaving, or passing through the shield. Metallic RF shields, commonly referred to as `shield cans," that separate RF circuits from one another and provide isolation between signal paths are well known in the art. RF shields are generally constructed in two parts--i.e., a thin continuous metallic wall that surrounds the circuitry and a cover that extends over the RF circuitry and attaches to the continuous metallic wall. The shield wall, commonly referred to as a fence, is typically soldered to a printed circuit board, or printed wiring board (PWB), and connected to the electrical ground of the board. In one implementation, the cover is held attached to the fence with copper tape. The copper tape retains the cover to the fence and prevents RF leakage as it seals any gap between the fence and the cover. This process, however, is both expensive and labor intensive.
In another implementation the cover is soldered to the fence creating a uniform seal around the RF circuitry, and insuring electrical contact with the electrical ground. Soldered shield covers provide excellent isolation, however the soldered shield cover is extremely difficult to remove when components within the enclosure require servicing. An example of this fixed attachment is U.S. Pat. No. 5,687,470 (Method for Forming an RF Shielded Enclosure) to Halttunen, et al., which discloses attaching an RF Shield cover with a conductive adhesive paste.
Other designs known in the prior art employ a cover with an openable lid to allow for the servicing of the components within the enclosure. U.S. Pat. No. 5,614,694 (One Piece Open and Closable Metal RF Shield) to Gorenz, et al., discloses such an enclosure formed from a single piece of conductive material in which the lid is hinged to the fence and swings open at a hinged point.
Other designs are known which employ removable covers. U.S. Pat. No. 5,365,410 (Electromagnetic Compatibility Enclosure) to Lonka, discloses opening the cover with a suitable tool and soldering it back or using a replacement cover. Other designs use compressive forces to grip the outer surface of the RF fence to retain the cover in place. This compressive force may typically be created by a set of tabs or finger-like projections that employ a spring-like compressive force to grip the outer surface of the fence.
The problem encountered with removable RF shield cover designs of this type is that the RF shield cover typically requires dedicated hard tooling to stamp a specific size and pattern of the cover and fingers from a thin metal. Typically metals having a width of 0.024 inches for fences and 0.015 through 0.020 inches for covers are used. Such thin metal for the cover is necessary to insure the tabs or fingers of the cover remain flexible and remain in contact with the shield walls. Attempts to fabricate the removable RF shield cover from thicker metal, for example, 0.024 inches, have proven to be not reliable and require expensive hard tooling. The tabs or fingers of these thicker covers are not resilient enough to guarantee good electrical contact with the fence walls. The thicker fingers are also prone to bending during assembly and do not spring back sufficiently to contact the fence wall. Thus, the effectiveness of the RF suppression is compromised because adequate contact is not maintained along the entire length of the fence. RF effectiveness is also compromised as the compressive forces of the fingers also tend to deflect the thin-walled fences inward. This deflection causes gaps between the fence and cover from which RF emissions can enter or escape the enclosure.
Thus, there is a need to provide a means of attaching RF shield covers to RF fences that allow for the simple removal of the cover without special tooling while providing effective suppression of RF emissions.